1. Field of the Invention
The present invention relates to a Light Emitting Diode (LED), and in particular, to a fabrication method of LEDs incorporating a step of surface-treating a substrate by a laser and an LED fabricated by such a fabrication method. More particularly, the present invention can use a laser in order to implement finer surface treatment to an LED substrate over the prior art thereby improving the light extraction efficiency of an LED while protecting the substrate from chronic problems of the prior art such as stress or defects induced from chemical etching and/or physical polishing.
2. Description of the Related Art
In general, nitride semiconductors such as InAlGaN are widely used for Light Emitting Diodes (LEDs) for realizing blue or green light. The nitride semiconductors have a representative formula AlxInyGa(1−x—y)N, wherein 0≦x≦1, 0≦y≦1, 0≦x+y≦1. A nitride semiconductor is fabricated by growing nitride epitaxial layers including an n-cladding layer, an active layer and a p-cladding layer on a substrate of for example sapphire via Metal Organic Chemical Vapor Deposition (MOCVD).
The light emitting efficiency of an LED is determined by an internal quantum efficiency, which represents light quantity generated by the LED with respect to external voltage, and an external quantum efficiency, which is measured outside the LED. Herein external quantum efficiency is expressed by the multiplication of internal quantum efficiency with light extraction efficiency. Therefore, it is essential to improve not only internal quantum efficiency but also external quantum efficiency in order to raise the light efficiency of the LED. In general, internal quantum efficiency is determined by active layer structure and epitaxy layer quality, and external quantum efficiency is determined by material refractivity and surface flatness.
Flip-chip LEDs have been increasing in their use. In a flip-chip LED, light generated from an active layer is emitted to the outside through a substrate of for example sapphire. Therefore, the external quantum efficiency of the flip-chip LED is determined by the interfacial state between a substrate and a buffer layer or an n-cladding layer and the outer surface state of the substrate.
Problems occurring in such a flip-chip LED will be described with reference to FIG. 1. As shown in FIG. 1, a fabrication process of a flip-chip LED 10 includes growing an n-GaN layer 14, an active layer 16 and a p-GaN layer 18 in their order on a substrate 12 for example sapphire and then etching a resultant structure into a mesa structure to expose a partial area of the n-GaN layer 14. Then, a p-electrode 20 is formed on the p-GaN layer 18, and an n-electrode 22 is formed on the exposed partial area of the n-GaN layer 14. Preferably, the p-electrode 20 is designed to cover the p-GaN layer 18 as large as possible so as to reflect light generated by the active layer 16 toward the sapphire substrate 12. The p-electrode 20 is properly made of Ag or Al having high reflectivity, and more preferably, made of Ag. The completed LED 10 having the electrodes 20 and 22 is mounted on a board via solder bumps 24 and 26 made of conductive paste, and electrically connected with patterns of the board.
However, such a flip-chip LED 10 has following problems. As shown in FIG. 1, when a light beam L1 is introduced into the sapphire substrate 12 directly from the active layer 16 or after reflecting from the p-electrode 20, total internal reflection takes place to the light beam L1 in a predetermined angle range owing to the refractivity difference between the n-GaN layer 14 and the sapphire substrate 12. Then, the light beam L1 is reflected several times between the sapphire substrate 12 and the reflective layer of p-electrode 20. In this way, the light beam L1 is absorbed and extinguished by the p- and n-GaN layers 14 and 18. This as a result causes light loss thereby to lower the light extraction efficiency and thus the external quantum efficiency of the flip-chip LED 10.
Various approaches have been proposed to solve these problems related with the light loss of such flip-chip LED 10. Representative examples may include Japan Patent Application Publication No. 2002-164296, (United States Patent Application Publication Nos. 2004-0038049 and 2004-0048471 both claiming the benefit of Japan Patent Application Publication No. 2002-164296) and Japan Patent Application Publication No. 2002-280611 (Unites States Patent Application Publication No. 2004-0113166 claiming the benefit of Japanese Patent Application Publication No. 2002-280611). These documents propose in common to roughen the interface between a substrate and an n-GaN layer in order to reduce light loss induced from the refractivity difference between the substrate and the n-GaN layer.
However, these approaches produce a roughened structure in common through chemical etching and thus disadvantageously have a difficulty in realizing a fine geometry. Since a substrate for example of sapphire is resistant to etching, harsh etching conditions are necessary. Under the harsh etching conditions, a photoresist having a pattern corresponding to a desired roughened geometry is also etched. As a result, those etching techniques using photoresists can hardly form fine surface geometries for example of pore or pillar size under 1 μm in substrates. Of course, it is much more difficult to uniformly produce a fine roughened geometry.
As another drawback of the above approaches, defects such as etching stress exist on the top of the roughened structure.
As a result, such etching-associated drawbacks cause nonuniform lighting to the flip-chip LED while degrading the efficiency thereof. In addition, substrates produced according to the above approaches need an additional process such as photolithography and dry etching to increase the entire process time thereby raising cost.
In the meantime, the flip-chip LED also has a following light loss problem, which will be described with reference to FIG. 2 as follows.
A flip-chip LED 10 shown in FIG. 2 has a structure substantially the same as shown in FIG. 1. When a light beam L2 generated in the LED 10 is directed in a predetermined angle range toward a sapphire substrate 12 instead of a p-electrode 20 functioning as a reflecting surface, the light beam L2 is reflected from the sapphire substrate 12 via total internal reflection owing to the refractivity difference between the sapphire substrate 12 and the air or external sealant such as silicone and resin. Then, the light beam L2 is reflected several times between the sapphire substrate 12 and the p-electrode 20. In this way, the light beam L2 is absorbed and extinguished by the sapphire substrate 12 and the n- and p-Gan layers 14 and 18. This as a result causes light loss to reduce the light extraction efficiency of the flip-chip LED 10 and thus the external quantum efficiency thereof.
Although it is desired to impart a roughened structure to the outer surface of the sapphire substrate 12 in order to overcome such a problem, a suitable approach has not been proposed up to the present. More specifically, a fabrication process of the LED 10 polishes the substrate 12 with a grinder containing for example diamond slurry to reduce the thickness thereof after forming the electrodes 20 and 22, and thus the outer surface of the substrate 12 can be roughened after the polishing. However, the foregoing etching cannot be performed to form the roughened structure in the outer surface after the formation of the semiconductor layers 12 to 18 and the electrodes 20 and 22.